Recognizing distinct portions of seamounts using volcanic facies analysis:examples from the Archean Slave Province, NWT, Canada

Archean volcanic rocks in the mafic-dominated, ca. 2.66-2.69 Ga Point Lake and Beaulieu River belts, Slave Province, Northwest Territories, are signif icant in demonstrating the facies that characterize specific portions of pi llow volcanoes or seamounts, irrespective of tectonic setting. Three distin ct localities mapped in detail display facies consistent with: (1) proximal , deep-water, (2) medial to distal. deep-water, and (3) medial, shallow-wat er seamount settings. The proximal facies in the Point Lake belt include a 55-m-thick, non-vesicular pillowed sequence cut by numerous mafic dykes and sills. Dykes contain multiple chilled margins, indicating successive magma pulses which contributed to edifice construction. Abundant feeder conduits , in addition to the absence of fragmental facies and vesicles, are typical of the central, deep water portion of seamounts where growth is initiated. The medial to distal, deep water facies in the Point Lake belt are represe nted by a 30-80 m-thick assemblage of disorganized pillow breccia, and pill owed and massive flows with 5-27% vesicularity. Massive, non-vesicular hyal oclastite intermingled with sedimentary material (fluidal peperite), in add ition to thin shale units interstratified with pillow breccia and hyaloclas tite, indicate that sedimentation and volcanism were contemporaneous. An in crease in fragmental units and vesicularity relative to the proximal, deep water facies is suggestive of the medial to distal part of a seamount in sh allower water. Bedded tuffs, laterally along strike with massive flows, are the results of turbidity current deposition immediately following localize d subaqueous eruptions. A medial, shallow water seamount setting is represe nted in the Beaulieu River belt, by a 5-85 m-thick sequence of vesicular lo bate-pillowed and massive hows, stratified pillow breccia and hyaloclastite , and mafic dykes. Vesicularity ranges from 21-49% in pillowed flows, 5-40% in massive flows, and 20-35% in pillow breccia and hyaloclastite. Stratifi ed pillow breccia developed along steep flow fronts in shallow water wherea s bedded hyaloclastite formed during reworking and redeposition of autoclas tic hyaloclastite on seamount flanks in shallow water. The volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the Mid-Atlantic Ridge and East Pacific Rise, as well as Me sozoic-Cenozoic seamounts in the Canary Islands, Fiji, southwest Japan, the Sea of Japan, and Cyprus. Volcanological studies in the feint Lake and Bea ulieu River volcanic belts and subsequent comparisons with Phanerozoic anal ogues, demonstrate the manner in which distinct portions of ancient seamoun ts can be recognized in similar Archean terranes. (C) 2000 Elsevier Science B.V. All rights reserved.